This disclosure pertains to microlithography (transfer of a fine pattern from a reticle to a substrate using an energy beam). Microlithography is a key technique used in the fabrication of microelectronic devices such as integrated circuits, displays, thin-film magnetic pickup heads, and micromachines. More specifically, the disclosure pertains to microlithography performed using a charged particle beam such as an electron beam or ion beam, and to controlling xe2x80x9cCoulombic blurxe2x80x9d (i.e., beam blur caused by the Coulomb effect) in apparatus and methods for performing charged-particle-beam microlithography.
A notable industrial trend continuing to this day is the progressive reduction in pattern linewidths of circuit elements of microelectronic devices, accompanying the progressively greater miniaturization and density of such devices as formed on semiconductor wafers and the like. Meanwhile, in processes for fabricating microelectronic devices, the linewidth-resolution limitations of optical microlithography have become increasingly apparent. These limitations provide ongoing motivation to the relevant industry to expend substantial effort in the development of a practical xe2x80x9cnext generationxe2x80x9d microlithography technology that offers prospects of substantially greater linewidth resolution than obtainable using optical microlithography. One candidate microlithography technology currently under intensive development is microlithography performed using a charged particle beam (e.g., an electron beam). A principal objective in this development effort is a charged-particle-beam (CPB) microlithography system that can achieve the desired high pattern resolution without substantially sacrificing throughput (number of wafers that can be processed lithographically per unit time).
An overview of the projection-optical system of a conventional electron-beam microlithography system, configured for projecting the pattern of a xe2x80x9cdividedxe2x80x9d or xe2x80x9csegmentedxe2x80x9d reticle, is shown in FIG. 5. An electron xe2x80x9cillumination beamxe2x80x9d is emitted from an electron-beam source situated upstream of the depicted projection-optical system. The illumination beam passes through an upstream illumination-optical system (not shown), which shapes and directs the illumination beam to illuminate a selected portion of a pattern-defining reticle 1. As the illumination beam passes through the illuminated portion of the reticle 1, the beam acquires an aerial image of the illuminated portion of the pattern. Thus, the beam propagating downstream of the reticle 1 is termed a xe2x80x9cpatterned beamxe2x80x9d 2. The patterned beam 2 passes through a projection-optical system comprising first and second projection lenses 3, 4. The projection-optical system resolves the aerial image on the upstream-facing surface of a wafer 5 or analogous substrate. So as to be imprinted with the image, the upstream-facing surface of the wafer 5 is coated with a suitable xe2x80x9cresist.xe2x80x9d Note that the image formed on the wafer 5 is smaller than the corresponding illuminated region on the reticle 1, normally by an integer factor (e.g., xc2xc, by which is meant that the image is xc2xc the size of the corresponding illuminated region on the reticle). This factor is termed the xe2x80x9cdemagnification ratioxe2x80x9d of the projection-optical system.
As noted above, the reticle 1 is xe2x80x9csegmentedxe2x80x9d or xe2x80x9cdividedxe2x80x9d by which is meant that the pattern on the reticle is divided into small exposure units, usually termed xe2x80x9csubfields,xe2x80x9d each defining a respective portion of the overall pattern. By way of example, each subfield conventionally is approximately 1 mm square in size. The subfields normally are arrayed in rows and columns on the reticle 1. By way of example, the width of a row of subfields is approximately 20 mm or other dimension representing the maximum distance over which the illumination beam can be deflected laterally (relative to the optical axis) for illumination of subfields. The subfields normally are illuminated sequentially subfield-by-subfield in each row, and row-by-row. This exposure sequence can be continuous (by continuous scanning) or intermittent (by step-and-repeat or step-and-scan). As each subfield is illuminated, the corresponding image, carried by the patterned beam 2, is formed on the surface of the wafer 5. By way of example, if the subfield size on the reticle 1 is 1 mm square, then (with a demagnification ratio of xc2xc) the size of the corresponding image as formed on the wafer surface is 0.25 mm square.
As the pattern is being illuminated subfield-by-subfield by the illumination beam, a deflector in the projection-optical system deflects the patterned beam 2 as required to form each subfield image at the appropriate location on the wafer surface. By forming the subfield images at the appropriate locations, the images are xe2x80x9cstitchedxe2x80x9d together properly to form a contiguous pattern. At a demagnification ratio of xc2xc, the width of each row of subfields as projected on to the wafer from a 20-mm row on the reticle is approximately 5 mm.
In the conventional CPB microlithography system described above, whenever a large exposure current is used, the patterned beam exhibits a problematical xe2x80x9cblurxe2x80x9d that arises from the so-called xe2x80x9cCoulombxe2x80x9d effect (mutual repulsion of like-charged particles in the beam). This xe2x80x9cCoulombic blurxe2x80x9d tends not to be uniform across the pattern as projected onto the wafer. Also, greater distortion is exhibited with greater degrees of lateral beam deflection. Coulombic blur results in degraded pattern resolution, critical-dimension (CD) uniformity, and overlay accuracy, as well as stitching errors from subfield to subfield. Conventional approaches to preventing or reducing Coulombic blur involve reducing the beam current used for making the exposure. However, an undesirable consequence of reducing the exposure current is a corresponding increase in the amount of time to expose each subfield. This causes a corresponding reduction in throughput.
In view of the deficiencies of conventional methods and apparatus as summarized above, the present invention provides, inter alia, charged-particle-beam (CPB) microlithography methods and systems that produce and exhibit, respectively, reduced Coulombic blur. These benefits are achieved without having to reduce beam current for exposure and without having to sacrifice exposure accuracy and throughput.
To such ends and according to an aspect of the invention, microlithography apparatus are provided that comprise a CPB optical system situated and configured to transfer-expose a pattern, divided into multiple exposure units each defining a respective portion of the pattern, onto a substrate using a charged particle beam passing through the CPB optical system. The CPB optical system is configured to transfer-expose the exposure units such that each exposure unit as imaged on the substrate has a maximum lateral dimension of at least 1 mm. The CPB optical system also is configured to provide the charged particle beam with a beam half-angle on the substrate of no more than 1 mrad.
The xe2x80x9cmaximum lateral dimensionxe2x80x9d referred to above can be a length or width (whichever is greater) of a square or rectangular exposure unit as exposed on the wafer. Hence, in this instance, the maximum lateral dimension is the maximum length of a line connecting two comers of the outline of the exposure unit. The xe2x80x9cexposure unitxe2x80x9d is the unit of the pattern that is exposed in one shot. If the exposure unit is not square or rectangular, but rather is some other polygonal shape, then the xe2x80x9cmaximum lateral dimensionxe2x80x9d is the longest diameter of the polygon as exposed on the wafer.
In the microlithography apparatus summarized above the exposure units can have different mutually perpendicular lateral dimensions. I.e., these exposure units are rectangular, wherein the width is different than the length of each rectangle. By configuring the exposure units in this manner, the beam distribution is widened, with a corresponding reduction in the Coulomb effect. Also, with respect to a divided reticle, by making the width of each exposure unit (i.e., dimension in the lateral beam-deflection direction) greater than the length of the exposure unit, the required magnitude of beam deflection necessary to expose the exposure units is reduced, with a corresponding reduction in deflection aberrations.